A transponder of the aforementioned type is presented in particular in the article “Fully Integrated Passive UHF RFID transponder IC with 16.7 μW minimum RF input power”, IEEE Journal of Solid-State Circuits, vol. 38, No 10, October 2003.
A similar transponder is also disclosed in U.S. Pat. No. 6,549,064.
The voltage rectifying and multiplying means for the signal received by the transponder disclosed in the aforementioned article are shown in FIG. 1 annexed hereto. These means comprise 2n+2 diodes in series, n being the number of voltage multiplication stages, and two series of N+1 capacitances, C1 to Cn+1 and C′n+1, arranged in parallel on either side of the diodes.
This type of voltage multiplier has significant drawbacks which decrease the output voltage actually provided and increase energy dissipation, which thus limits the communication distance between a reader and the transponder.
It is an object of the present invention to provide a transponder, arranged for receiving very high frequency signals, in particular UHF signals, which can communicate with a reader or a transceiver, transmitting at a given power, at a greater distance than that of the aforecited transponders of the prior art.
Within the scope of current developments, the inventors have observed that the analogue part of the integrated circuit defines an input capacitance, which is coupled to the inductive antenna. This input capacitance and the antenna together form a resonant circuit that dissipates a certain amount of energy proportional to the frequency of the received electromagnetic signal. An antenna has a certain aperture enabling it to pick up a certain amount of energy from the reader's electromagnetic field that reaches the antenna. The energy received by the transponder is dissipated in said resonant circuit and is also consumed by the integrated circuit. For a given quality factor, the energy dissipated by the resonant circuit is proportional to the energy stored therein. This stored energy is equal to 0.5×CE×VP2, CE being the input capacitance and VP the maximum voltage at the terminals of said input capacitance. Thus, in order to decrease the dissipated energy, CE should be decreased. For a given VP, by decreasing the input capacitance, one can thus decrease the energy that has to be picked up by the antenna in order for the transponder to work. This is what the present invention proposes. This results in an increase in the communication distance between a given reader and the transponder.
It will also be noted that the lower the input voltage VP is kept for a determined rectified voltage at the terminals of the transponder logic circuit, the greater the communication distance can be. The voltage rise contributes to obtaining this effect. The more efficient the voltage rise, the greater the communication distance.
The received signal voltage rectifying and multiplying means proposed in the prior art of FIG. 1 have a relatively high input capacitance due in particular to the capacitance series C2 to Cn+1, which is added to capacitance C1 of an input voltage rectifier circuit.
Next, diodes D3 to D2n+1 are floating diodes with the anode or cathode thereof subjected to a relatively significant alternating signal generated by the received RF signal. As shown in FIG. 2, odd coefficient diodes D2n+1, N=1 to n, each have, in a standard CMOS monolithic integration, a stray capacitance CPN with the substrate which decreases the voltage rise through said diodes. This results in a decrease in the efficiency of the voltage booster, which is a major drawback.
In order to overcome the aforementioned drawbacks, the present invention proposes a passive transponder comprising alternating voltage rectifying and multiplying means for the received electromagnetic signal, which is characterized in that said means comprise a passive rectifier supplying a rectified voltage and a voltage multiplier or booster arranged downstream of the passive rectifier and formed by capacitors switched to a lower frequency than said received signal frequency.
Preferably, the capacitors are switched at a relatively low frequency using transistors defining active switches.